Occupancy sensors are used to monitor the presence of human occupants in indoor and outdoor spaces. Occupancy sensors conserve energy by automatically turning off lighting and other electrical loads for a space when the space is unoccupied. Occupancy sensors also perform a convenience function by automatically turning on lighting and other loads when an occupant enters a given space.
Sensing technologies used with occupancy sensors can generally be characterized as either active or passive technologies. Passive technologies do not involve the active emission of any type of energy in the monitored space. Instead, passive technologies rely on the detection of energy given off by the occupants themselves, or reflected by the occupants from ambient sources. An example of a passive occupancy sensing technology is passive infrared (PIR) sensing. PIR sensing relies on the fact that the thermal energy of warm objects causes them to emit infrared radiation. The infrared radiation is sensed by a photocell which converts the radiation to electric signals for further processing. Another type of passive occupancy sensing technology is video sensing which relies on ambient light that is reflected by an occupant and detected by a video sensor such as a charge coupled device (CCD).
With active technologies, some type of energy is emitted in the monitored space. The emitted energy is reflected by an occupant and converted into an electric signal by a suitable sensor. An example of an active occupancy sensing technology is ultrasonic sensing. In an ultrasound system, the monitored space is flooded with ultrasonic waves that are constantly emitted by an ultrasound driver. An ultrasound sensor detects waves that are reflected by an occupant and/or other objects in the monitored space. By comparing the emitted and reflected waves, an ultrasonic system can determine whether an object is moving. Moving objects are assumed to be occupants.
Some occupancy sensors use a combination of sensing technologies. For example, PIR is generally more accurate for detecting large motion such as a person walking into a room in a path that is directly within the line-of-sight of the occupancy sensor. Ultrasound systems tend to be more sensitive for detecting small motion, such as a person working at a desk, and motion that is hidden from the line-of-sight of the occupancy sensor, such as behind partitions in an office or restroom. The added sensitivity, however, may cause false occupied readings. Therefore, an occupancy sensor may initially use only PIR sensing to determine that the monitored space has become occupied. Once the space is initially determined to be occupied, an occupied reading from either PIR or ultrasound may be used to determine that the space continues to be occupied. A countdown timer is typically used to keep the lights on only for a predetermined period of time unless occupancy is sensed again during the countdown time. The countdown timer is reset to a predetermined value (typically 10-30 minutes) when occupancy is initially sensed and the lights are turned on. The timer then continues to decrement toward zero. Each occupied reading from either PIR or ultrasound causes the timer to reset to the maximum value. If the timer decrements all the way to zero before another occupancy event is detected, the lights are turned off, and the sensor returns to the PIR only sensing mode.